1. Field of the Invention
The present invention relates to an apparatus for detecting a characteristic of a recording material using an ultrasonic wave.
2. Description of the Related Art
A conventional image forming apparatus enables a user to set a type of a recording material (which may be referred to as “paper type”) via an operation panel provided on as an external apparatus, such as a computer, or on the body of the image forming apparatus. The image forming apparatus can be controlled in such a way as to optimize image transfer conditions (e.g., bias, an image forming speed) and image fixing conditions (e.g., a quantity of heat, a recording material conveyance speed) according to the settings.
However, if a user forgets or fails in setting a type of a recording material, it is unable to optimize image forming conditions. A defective image may be generated. Therefore, to reduce a burden on a user or to appropriately control the image forming conditions according to a type or a state of a recording material, it is conventionally known that a recording material detection sensor can be provided in an image forming apparatus to automatically determine the type of the recording material.
More specifically, Japanese Patent Application Laid-Open No. 2007-24837 discusses a method in which a grammage of a recording material can be determined based on a transmittance value of an ultrasonic wave that penetrates through the recording material. Japanese Patent Application Laid-Open No. 2010-18433 discusses a method in which a rising waveform of an ultrasonic wave reception unit is used in grammage detection. When the method discussed in Japanese Patent Application Laid-Open No. 2010-18433 is employed, the grammage of a recording material can be accurately detected based on ultrasonic waves, while the positional variation of the recording material can be suppressed and reflections from members surrounding the sensor or environmental variations can be reduced.
According to the above-described detection method using ultrasonic waves discussed in Japanese Patent Application Laid-Open No. 2007-24837 or Japanese Patent Application Laid-Open No. 2010-18433, it is necessary to correct a change in the propagation speed of ultrasonic waves in an environment in which an ultrasonic wave detection apparatus is installed to accurately detect the grammage of a recording material. For example, to obtain the grammage of a recording material, a reception signal (P0) in a state where no recording material is present between an ultrasonic wave transmission unit and an ultrasonic wave reception unit and a reception signal (P1) in a state where a recording material is present between the ultrasonic wave transmission unit and the ultrasonic wave reception unit are obtained, and the grammage is detected based on a ratio of the reception signal P0 to the reception signal P1 (i.e., ratio P1/P0). The propagation speed of ultrasonic waves is variable depending on environmental factors. Therefore, to accurately obtain the detection timing of each of the reception signals P0 and P1, a correcting operation needs to be performed according to the propagation speed of ultrasonic waves considering the environmental factors.